FIG. 1 illustrates at 10 the concept of voltage boosting. The purpose of a voltage booster is to generate a specified voltage higher than VDD at node V-boosted, where VDD is the power supply voltage. A voltage booster basically contains two parts: a pre-charge circuit and a boosting capacitor (C boost). Before node Boost_Ctl is pulled to high, the pre-charge circuit charges node Ncb to some positive voltage. Then, node Boost_Ctl is pulled to VDD and switch S1 is turned on to charge load capacitor C_load to a voltage level above VDD. This boosted voltage level is determined by the voltage at node Ncb before Boost_Ctl goes high and the ratio of C_boost/C_load. The lower the pre-charged voltage at node Ncb, the larger the capacitance ratio is needed, and the larger the area cost for capacitor C_boost. Thus, it is best to pre-charge node Ncb to VDD before Boost_Ctl starts going high.
There are four ways known in the prior art of voltage boosters to pre-charge C_boost to VDD:
1) As shown in U.S. Pat. Nos. 5,999,461 and 4,186,436, when the boosted voltage is needed, the pre-charge circuit is enabled, but Boost_Ctl is not pulled to high until node Ncb is charged to VDD. Obviously, the major disadvantage is that some delay must be introduced. Thus, this scheme is not applicable to high speed devices.
2) As shown in U.S. Pat. Nos. 6,268,761 and 6,275,425, PMOS transistors are used to pre-charge node Ncb to VDD and keep the voltage at VDD during standby. The disadvantage is that the sizes of the PMOS transistors are huge when capacitor C_boost is large and pre-charging must be completed in a short time.
3) As shown in U.S. Pat. Nos. 5,175,448 and 5,636,115, NMOS transistors with their gate boosted above VDD are used to pre-charge node Ncb to VDD during standby. The advantage of using NMOS transistors is that the sizes of the transistors can be much smaller due to the higher drive capability of NMOSFET than PMOSFET. However, the voltage at node Ncb cannot be kept at VDD for a long time due to various leakages. When the voltage at node Ncb decreases, the size of capacitor C_boost must be increased in order to boost the voltage to the same level.
4) As shown in U.S. Pat. No. 5,701,096 charge pumps are used to continuously supply charge to capacitor C-boost and keep the voltage at some level. However, complex timing and control circuits must be introduced.
In one conventional memory application, a voltage booster is needed to boost the word line above VDD to VDD+Vthn, and the memory access time is about 11 ns. This means that there is no time for pre-charging the boosting capacitor after the chip is enabled, and the cycle time is about 20 ns. Thus, the boost capacitor must be quickly pre-charged to VDD as soon as the memory access is terminated. In the technique described in 3) above, the NMOS transistors are used as pre-charge devices. The boost capacitor is huge (60 pf) because the voltage on the booster capacitor decreases to a level below VDD due to the leakages. Furthermore, it takes a long time to pre-charge the boost capacitor to VDD due to the huge size.